Outlet Surface Type as Extrapolated

One of the types for an outlet is "Extrapolated". What does that mean?

I applied to a box with a fan at one end and the extrapolated outlet and pressure at both ends was almost equal.


Extrapolated pressure outlet not useful for subsonic flow

I haven't found the extrapolated outlet (it refers to extrapolating pressure) to be useful for subsonic flow. I'd suggest you use the default outlet (fixed pressure).